In the 50s, when I began playing bass, almost everything available was a Kay. I had about half a dozen of them through the years, and formed something of an opinion, though it's not set in cement.
C -1's, set up right, were nice little basses for all -around purposes. But they were always equipped with gut top strings, and usually copper wound bottoms (very expensive the last time I checked) - they woldn't stay in tune through one piece of music. I think the survivors should be good chamber basses, with new set ups. (I have used Thomastic strings exclusively through the years).
The S series seemed to be extremely variable in tone, resonance, and all that. I had one S series bass with a wonderful singing tone that the audiences (not necessarilly myself) liked, especially bowed, with things like Galliard, etc sonatas. It was a pretty good bass for as long as I had it. The swingmasters, unampliified, also varied all over the place, but I never heard or played one with much volume. Designed for appearances more than anything else. Chubby Jackson five strings have always sounded "blunted" to me - like they couldn't quite let it all hang out. Half way between decent and so - so. I had the oppostunity to get one for $2000 about ten years ago and declined.
Lots of sunken tops on old Kays. Most of them died before their twentieth birthdays. But some survived well, and now sound pretty as orchestra basses, at least. I figure that if a Kay lasted 2 decades, it was going to be alright. And I think that's about how it seems to have worked out. A lot of that has to do with maintenance and life style, too. Most of them were used a bit on the rough side. The few that held up are pretty nice basses - IMO.
I'd love to get a present of a decent C-1, place it among my memorabila and play it every now and then. At todays price for a C-1 - there's little likelihood of that. Should have kept my forst one!